I heard the Prophet ﷺ saying, "If someone keeps a dog neither for hunting, nor for guarding livestock, the reward (for his good deeds) will be reduced by two Qirats per day."  

سَمِعْتُ النَّبِيَّ ﷺ يَقُولُ مَنِ اقْتَنَى كَلْبًا إِلاَّ كَلْبٌ ضَارٍ لِصَيْدٍ أَوْ كَلْبَ مَاشِيَةٍ فَإِنَّهُ يَنْقُصُ مِنْ أَجْرِهِ كُلَّ يَوْمٍ قِيرَاطَانِ  

Messenger of Allah ﷺ said, "Whoever keeps a dog, one Qirat of the reward of his good deeds is deducted daily, unless the dog is used for guarding a farm or cattle."  

قَالَ رَسُولُ اللَّهِ ﷺ «مَنْ أَمْسَكَ كَلْبًا فَإِنَّهُ يَنْقُصُ كُلَّ يَوْمٍ مِنْ عَمَلِهِ قِيرَاطٌ إِلاَّ كَلْبَ حَرْثٍ أَوْ مَاشِيَةٍ۔»